by Jon Swartz, USA TODAY

by Jon Swartz, USA TODAY

SAN FRANCISCO - The $35.1 billion monster merger that Omnicom Group and Publicis announced Sunday could send rumblings throughout the digital-advertising landscape.

While Google is the world's biggest digital-ad network - and Facebook has shown encouraging gains in mobile advertising, based on its financial results July 25 - the specter of OmniPub, as some call it, could eventually change all that.

The combined company, expected to be called Publicis Omnicom Groupe, will create an advertising megacorporation that would be the world's biggest provider of advertising.

Publicis has been especially attentive to scooping up digital-ad firms, such as Razorfish and Digitas, establishing it as a fledgling player in that space. Omnicom, meanwhile, has preferred to work with technology companies and build internal web capability.

The deal can only cause consternation at Google, Facebook, Yahoo, Apple and other tech titans fighting for a piece of the multibillion-dollar online advertising pie. For many, success in online ad selling is the Holy Grail for long-term success.

"The digital focus makes sense," says Clark Fredricksen, vice president at eMarketer. "It's indicative of both direction consumer behavior is trending and where advertisers are increasingly investing."

One of the key challenges for each is finding space on mobile devices to insert ads without offending consumers and cluttering content. Google has done a masterful job of parlaying its search results into ad riches and Facebook seems to be bucking the mobile quandary because its users don't seem to mind the tiny ads. Yahoo, too, has made strides in reinventing itself under new CEO Marissa Mayer with acquisitions.

"Google, Twitter, Facebook and others will have to be somewhat more accommodating to the new, giant agency," says Greg Sterling, an analyst who closely follows social media. The mega-merger could yield more mergers as tech players feel pressure to get bigger, he says.

"That will give additional power back to the brands and agencies vs. the big networks and sites," Sterling adds.

But with the emergence of the world's largest ad agency, and its piqued interest in digital advertising, these major tech players now have another competitor - far larger, and with greater resources - to worry about.